Protective arrangements for downhole tools

ABSTRACT

A protective arrangement or &#34;junk bonnet&#34; (30) for preventing debris entering a downhole tool assembly (14) employed in the drilling of a well. The junk bonnet (30) is in the form of an annular seal which fits around a tube (18) and inside a sleeve (24) which are relatively movable to operate a downhole tool assembly (14) of which they form part. The junk bonnet (30) is provided with sliding fluid seals (42, 44, 48, 50) which seal against the external surface (19) of the tube (18), and against the bore (25) of the sleeve (24). A longitudinal fluid passage (52) links opposite ends (54, 56) of the junk bonnet (30). The passage (52) is sealed in use by a plug (60) or other suitable valve. An annular volume (&#34;C&#34;) below the junk bonnet (30), between the tube (18) and the sleeve (24), and above a seal (26) forming part of the downhole tool assembly (14) is filled through the passage (52) with hydraulic oil during setting-up and prior to closure of the passage (52) by the plug (60). This hydraulically locks the junk bonnet (30) in place on the downhole tool assembly (14), without reliance on any mechanical fastening, and yet free of susceptibility to being dislodged by normal operational movements of the downhole tool assembly (14). The upper end (56) of the junk bonnet (30) can be formed as a back-reamer, and an emergency release (72) can be incorporated.

FIELD OF THE INVENTION

This invention relates to protective arrangements, and relates moreparticularly but not exclusively to protective arrangements for downholetool assemblies employed in the drilling of wells.

In the drilling of wells intended for exploration of subterraneanformations and/or the eventual recovery of hydrocarbons or otherminerals, much of the work involves drilling of the well. However, fromtime to time it is necessary to lower a tool assembly down the well forthe accomplishment of related operations. For example, a liner runningand cementing operation may be necessary in the course of drilling awell. Such tool assemblies have varying degrees of complexity, andfrequently involve fluid flow passages and/or relatively movablecomponents or sub-assemblies.

DESCRIPTION OF THE PRIOR ART

Proper functioning of such tool assemblies can be disrupted by the entryof drilling debris into the mechanism or fluid passages. A form ofprotective arrangement for obviating such debris-induced disruption isknown as a "junk bonnet", and takes the form of a cap intended to shieldthe tool assembly from descending debris. However, known forms of junkbonnet do not provide a fluid seal, and are liable to be dislodged bymovement of the drill pipe as part of operation of the tool assembly.These problems allow debris to enter the tool assembly, and result infailure of the protective arrangement. Further, the known forms of junkbonnet have a relatively large outside diameter which restricts fluidcirculation in the annulus between the drill pipe and the casing, andwhich hinders easy removal of the tool assembly from the well when alarge amount of cuttings and drilling debris has accumulated in theannulus.

It is therefore an object of the invention to provide a protectivearrangement for a downhole tool assembly which obviates or mitigatesthese problems.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention there is provided adownhole tool assembly comprising a sleeve member, a tube member axiallymovable with respect to and at least partially located within saidsleeve member, a seal member mounted within a lower end of said sleevemember providing a sliding fluid seal to the exterior of said tubemember or to the exterior of a tubular extension member thereof andhaving an external diameter substantially equal thereto, a downhole toolsub-assembly and a protective arrangement comprising an annular sealmeans characterised in that said annular seal means has an inner surfacedimensioned to fit around said tube member and an outer surfacedimensioned to fit within at least an upper end of said sleeve member,said inner surface being provided with an inner fluid seal means forforming a sliding fluid seal to the exterior of said tube member, saidouter surface being provided with an outer fluid seal means for forminga sliding fluid seal to the interior of said sleeve member and thedownhole tool sub-assembly being located within an annular Volume (C)between the seal member and said annular seal means, the Volume (C)being constant nothwithstanding movement of the tube member relative tothe sleeve member.

The annular seal means preferably includes a fluid passage linkingaxially opposite ends of said annular seal means, and fluid flow controlmeans selectively operable either to open said fluid passage to the flowof fluid therethrough between said axially opposite ends of said annularseal means or to close said fluid passage to the flow of fluidtherethrough between said axially opposite ends of said annular sealmeans.

Said fluid flow control means may comprise a valve means fitted in saidfluid passage or on one end thereof; said fluid flow control means morepreferably comprises a plug insertable into said fluid passage forretention therein to block said fluid passage to fluid flow therethroughbetween said axially opposite ends of said annular seal means.

Said annular seal means preferably comprises a body which is axiallydivided into a plurality of mutually securable segments whereby saidannular seal means may be assembled around a selected portion of saidtube member of said tool assembly. Thereby said annular seal means neednot be threaded onto a free end of said tube member during setting up ofsaid protective arrangement as would be the case if the body of saidannular seal means were in one piece. More preferably, said body isformed as a pair of hemi-annular segments which can be mutually securedby latches, clamps, bolts or any other suitable securing means. Gasketsmay be incorporated between mating faces of said body segments toinhibit fluid leakage.

The axial end of said annular seal means which is uppermost and remotefrom said tool assembly in use of said protective arrangement ispreferably formed as a reamer or debris cutter to facilitate withdrawalof said protective arrangement from a downhole location subsequent touse of said tool assembly.

The axial end of said annular seal means which is lowermost and adjacentsaid tool assembly in use of said protective arrangement is preferablyformed with dog clutch means selectively engageable with matching dogclutch means on said tool assembly to enable said annular seal means tobe selectively rotated by rotation of said tube member of said toolassembly during withdrawal of said protective arrangement from adownhole location subsequent to use of said tool assembly.

Said inner fluid seal means may comprise at least one elastomeric ringdisposed in and standing proud of a circumferential slot or grooveextending circumferentially around said inner surface, and morepreferably comprises a plurality of elastomeric O-rings each disposed inand standing proud of a respective circumferential slot or groove eachextending circumferentially around said inner surface.

Said outer fluid seal means may comprise at least one elastomeric ringdisposed in and standing proud of a circumferential slot or grooveextending circumferentially around said outer surface, and morepreferably comprises a plurality of elastomeric O-rings each disposed inand standing proud of a respective circumferential slot or groove eachextending circumferentially around said outer surface.

According to a second aspect of the present invention there is provideda method of assembling and setting up the protective arrangementaccording to the first aspect of the present invention in conjunctionwith a downhole tool assembly of the kind specified, said methodcomprising the steps of providing said annular seal means and saidmembers of said tool assembly, threading said tube member or saidtubular extension member thereof (if present) through said seal membermounted in said sleeve member, locating said annular seal means aroundsaid tube member and inserting said annular seal means into said upperend of said bore of said sleeve member while said fluid passage isinitially open so as to form an annular volume bounded externally bysaid bore of said sleeve member, bounded internally by the exterior ofsaid tube member and by the exterior of said tubular extension memberthereof (if present), bounded at one end by said seal member, andbounded at the other end by said annular seal means, filling saidannular volume with a hydraulic liquid such as to exclude substantiallyall gasses from said annular volume, and operating said fluid flowcontrol means to close said fluid passage to the passage of fluidtherethrough between said axially opposite ends of said annular sealmeans whereby said annular seal means is retained in said upper end ofsaid sleeve member substantially only by hydraulic forces resulting fromthe substantial invariability of said annular volume upon relative axialmovement of said tube member and of said tubular extension memberthereof (if present) with respect to said sleeve member and said sealmember mounted therein.

According to a third aspect of the present invention there is provided acombination of a protective arrangement according to the first aspect ofthe present invention and a downhole tool assembly of the kindspecified, assembled and set up by the method according to the secondaspect of the present invention.

Said downhole tool assembly preferably incorporates an emergency releaseassembly in said tube member or in said tubular extension member thereof(if present) or therebetween (for example, in a further component orsub-assembly of said tool assembly), said emergency release assemblypreferably comprising a coupling means mechanically and hydraulicallycoupling parts of said tool assembly above and below said couplingmeans, said coupling means incorporating shear means rupturable to partsaid coupling means by an upward force applied through said tube memberand in excess of a predetermined normal maximum upward force prevailingduring non-emergency operation. Said coupling means preferablyincorporates a vent opened by parting of said coupling means to ventsaid annular volume and so hydraulically release said annular seal meansfrom its normal hydraulic retention in said upper end of said sleevemember.

Said tube member preferably incorporates a longitudinal seal bypassmeans at a location on the exterior surface thereof normally below saidannular seal means but capable of being lifted, by upward movement ofsaid tube member with respect to said annular seal means, to a higherlocation in which said longitudinal seal bypass means longitudinallybypasses said inner seal means of said annular seal means to vent saidannular volume and so hydraulically release said annular seal means fromits normal hydraulic retention in said upper end of said sleeve member.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described by way ofexample with reference to the accompanying drawing, the sole FIGURE ofwhich is a longitudinal elevation of a length of casing and hanger/linerenclosing a downhole tool assembly incorporating a protectivearrangement in accordance with the invention, the right half of theFIGURE being radially sectioned as far as the centre-line of theassembly, and the left half of the FIGURE being conventionally radiallysectioned (is to show the radially external surface of the radiallyinnermost entities of the assembly).

DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to the FIGURE, there is shown a downhole section of previouslydrilled well lined with 95/8 inch (244.5 millimeter) casing 10. A 7 inch(177.8 millimeter) liner 12 is shown run within the casing 10 on thelower end of a liner running/cementing downhole tool assembly 14 ofknown form. The tool assembly 14 is suspended within the casing 10 onthe lower end of a drill pipe 16.

Those parts of the tool assembly 14 which are of significance to thepresent invention are a setting tool tube member 18 having an insidediameter of 71.4 inch millimeters and an external surface 19 with adiameter of 100 millimeters, a running tool sub-assembly 20 below thetube member 18, and a tubular stinger extension member 22 depending fromthe sub-assembly 20. The tubular extension member 22 of the toolassembly 14 has an external diameter of 100 millimeters, ie the sameexternal diameter as the external diameter of the tube member 18. Thetool assembly 14 further comprises a coaxial PBR (polished borereceptacle) sleeve member 24 to the lower end of which the liner 12 isattached, coaxially with the sleeve member bore 25. Mounted within thelower end of the sleeve member 24 is an annular seal 26 suspended on a 7inch (177.8 millimeter) nipple 28 forming the lower end of the sleevemember 24. The annular seal 26 forms a sliding fluid seal on the tubularexternal surface of the tubular extension member 22 of the tool assembly14.

To protect the tool assembly 14 from having its correct operationdisrupted by cuttings and other drilling debris falling into it, thetool assembly 14 is provided with a protective arrangement in the formof a junk bonnet 30 which is an annular seal assembly fitted around thetubular external surface 19 of the tube member 18 and within the tubularbore 25 at the upper end 32 of the s! eeve member 24 to provide asliding fluid seal to each of these surfaces.

The junk bonnet 30 comprises two axially elongate hemi-annular segments34 mutually secured by bolts 36, with gaskets 38 clamped between matingfaces of the segments 34 to inhibit end-to-end fluid leakage.

The inner surface 40 of the annular junk bonnet 30 is provided with alower inner seal 42 in the form of an elastomeric O-ring mounted withina circumferential slot extending around the inner surface 40, and isfurther provided with a pair of upper inner seals 44 each in the form ofan elastomeric O-ring mounted within a respective circumferential slotextending around the inner surface 40.

The outer surface 46 of the annular junk bonnet 30 is provided with apair of lower outer seals 48 each in the form of an elastomeric O-ringmounted within a respective circumferential slot extending around theouter surface 46, and is further provided with a pair of upper outerseals 50 each also in the form of an elastomeric O-ring mounted within arespective further circumferential slot extending around the outersurface 46.

Each of the O-ring seals 42, 44, 48, and 50 stands slightly proud of itsrespective mounting slot to form a sliding seal with the externalsurface 19 of the tube member 18 and with the bore 25 of the sleevemember 24, respectively. A longitudinal fluid passage 52 extends betweenand hydraulically links the lower axial end 54 of the junk bonnet 30with its upper axial end 56, for a purpose detailed subsequently. Atransverse fluid passage 58 links the longitudinal fluid passage 52 withthe inner surface 40 between the upper and lower inner seals 44, 42, andwith the outer surface 46 between the upper and lower outer seals 50,48, for a purpose also to be detailed subsequently. The longitudinalfluid passage 52 can be selectively blocked or opened for the passage offluid through the junk bonnet 30 between its axially opposite ends 54and 56, by the insertion or removal of a screw-threaded plug 60.

Setting up of the above arrangement will now be described.

At the well surface, components of the tool assembly 14 are put togetherin known manner, with the annular seal 26 mounted within the nipple 28in the lower end of the sleeve member 24, and the tubular stingerextension member 22 threaded therethrough to form a sliding fluid sealtherewith.

The pair of junk bonnet segments 34 are assembled around the tube member18 of the tool assembly 14, thereby avoiding potential problems inhaving to thread the junk bonnet 30 over the pipe connector 62 by whichthe tube member 18 will be attached in use to the drill pipe 16. (Theinner seal rings 42 and 44 can be fitted over the pipe connector 62,since these rings are elastic and circumferentially stretchable). Therings 42 and 44 are fitted into their slots on the inner surface 40, thegaskets 38 are placed between the mating faces of the segments 34 whichare then mutually clamped by the bolts 36, the outer seal rings 48 and50 are fitted into their slots on the outer surface 46, and thethus-assembled junk bonnet 30 is slid down the external surface 19 ofthe tube member 18 to be inserted into and rest partly within the bore25 at the upper end 32 of the sleeve member 24.

This arrangement forms an annular volume, denoted "C" in the FIGURE,which is externally bounded by the bore 25 of the sleeve member 24,internally bounded by the exterior surface 19 of the tube member 18 inconjunction with the exterior of the tubular extension 22 and the toolsub-assembly 20 linking them, bounded at the lower end by the annularseal 26, and bounded at the upper end by the lower axial end 54 of thejunk bonnet 30.

While the longitudinal fluid passage 52 through the junk bonnet istemporarily open by the absence of the plug 60, the annular volume "C"is filled with an appropriate oil or other suitable hydraulic fluid,such as to exclude all air from volume "C". (Total filling can beassisted by pushing the junk bonnet 30 down a short distance into theoil-filled sleeve member 24 such to force oil out of the passage 52).Volume "C" is then sealed by tight insertion of the plug 60.

Since the external diameters of the tube member 18 and of the tubularextension 22 are the same where they pass through the respective slidingfluid seals 44 and 26, longitudinal movement of the tube member 18together with the tool sub-assembly 20 and the tubular extension member22 with respect to the sleeve member 24 and the seal 26 mounted thereincauses no change in the volume of the annular volume "C". Thus the junkbonnet 30 is hydraulically locked in its position plugging the upper end32 of the sleeve member 24, without reliance on any mechanical fasteningor the like, and yet free of susceptibility to being dislodged by normaloperational lifting/lowering of the sub-assembly 20 within the downholetool assembly 14.

When the combination of the tool assembly 14 and the junk bonnet 30 (setup as described above) is coupled to the drill pipe 16 and lowereddownhole to an operational depth in the already-drilled well for theperformance of a drilling associated operation therein (liner runningand cementing in this example), the junk bonnet 30 prevents cuttings orother debris from dropping into the tool assembly 14, thus to serve as aprotective arrangement therefor.

In the downhole tool operating configuration illustrated in the FIGURE,fluid can be circulated down the bore of the drill pipe 16 and the boreof the tube member 18, and through the bores of the tool sub-assembly 20and of the tubular extension member 22, to discharge from the bottom(not shown) of the liner 12 and return uphole to the surface through theannulus between the tube member 18 and the casing 10. During suchcirculation, the volume "C" remains hydraulically sealed and protectedfrom debris. The minimal outside diameter of the junk-bonnet 30 (lessthan the outside diameter of the sleeve member 24) obviates obstructionof circulation in the annulus between the tube member 18 and the casing10.

Prior to cementing of the liner 12 in its intended place in the well,the liner running tool sub-assembly 20 is unscrewed from its initial (asset-up) screw attachment 64 to the sleeve member 24 of the downhole toolassembly 14 by anti-clockwise (as viewed from above) rotation of thedrill pipe 16, and lifted by a meter or so to check for proper releaseof the screw attachment 64. Such operation of the downhole tool assembly14 commonly causes entry of debris in prior art configurations of junkbonnet, but with the junk bonnet 30 of the present invention, suchdebris entry is entirely inhibited since the junk bonnet 30 fully sealsthe top of the sleeve member 24 and remains hydraulically locked inplace.

At the end of the liner cementing operation, the drill pipe 16 and therunning tool sub-assembly 20 are pulled from the well, leaving thesleeve member 24 and the liner 22 downhole. To achieve such withdrawal,it is necessary first to break the hydraulic locking of the junk bonnet30 in the upper end 32 of the sleeve member 24. To this end, alongitudinal hydraulic bypass slot 66 is cut in the exterior surface ofthe tube member 18, at a location thereon sufficiently far down as tohave no effect on setting-up, running, and cementing operations, buteffective on sufficient lift being imparted to the drill pipe 16 as torun up the inner surface 40 of the junk bonnet 30 to an extent that theslot 66 bypasses the inner seals 44, thus breaking the hydraulic lockand freeing the junk bonnet 30.

There is a known problem in well drilling operations that the top of thecemented-in liner may accumulate such an excess of debris that thebypass slot 66 would become blocked before it as effective to break theaforementioned hydraulic locking of the junk bonnet 30. To obviate ormitigate this hazard, the upper axial end 56 of the junk bonnet 30 isoptionally formed as a conical reamer, while the lower axial end 54 isformed with one or more notches 68 engageable by one or more matchingaxial lugs 70 on the upper end of the sub-assembly 20. The notches 68and the lugs 70 function as a dog clutch to enable the junk bonnet 30 tobe lifted and rotated to back-ream its way through the excess of debris.

To guard against continued jamming of the junk bonnet 30 despite suchprovision for back-reaming and/or failure of hydraulic lock breaking,the lower end of the tube member 18 is provided with an emergencyrelease coupling 72 which connects the tube member 18 with thesub-assembly 20. The coupling 72 comprises an external sleeve 74covering and hydraulically sealing the mutual junction of the tubemember 18 with the sub-assembly 20, and a radial port 76 hydraulicallylinking the normally isolated bore through the tool assembly 14 with theannular volume "C". The sleeve 74 mutually secures the mating halves ofthe coupling 72 by means of an array of shear screws 78 selected to partupon application trough the drill pipe 16 and the tube member 18 of apredetermined lift force substantially in excess of lift forces appliedin normal operation of the downhole tool assembly 14. Such overliftfractures the shear screws 78, parts the emergency release coupling 72,and vents volume "C" through the radial port 76. This breaks thehydraulic locking of the junk bonnet 30 and also provides additionalcirculation for debris clearance.

The transverse fluid passage 58 linking the fluid passage 52 and theinner and outer junk bonnet surfaces 40 and 46 allows testing of thejunk bonnet seals 42, 44, 48 and 50 on a suitable test rig (not shown)prior to setting-up and use of the junk bonnet 30.

The above described exemplary embodiment of downhole tool assembly andof the protective arrangement therefor have been described in thecontext of a substantially vertically drilled well. However, sucharrangements can also be employed in inclined and horizontal wells, andin analogous situations such as tunnelling and mining.

The present invention is not restricted to the exemplary embodimentsdescribed above, nor to the optional variations thereof also referredto, and other modifications and variations can be adopted withoutdeparting from the scope of the invention as defined in the appendedclaims.

I claim:
 1. A downhole tool assembly comprising a sleeve member (24), a tube member (18) axially movable with respect to and at least partially located within said sleeve member (24), a seal member (26) mounted within a lower end of said sleeve member (24) providing a sliding fluid seal to the exterior of said tube member (18) or to the exterior of a tubular extension member (22) thereof and having an external diameter substantially equal thereto, a downhole tool sub-assembly (20) and a protective arrangement comprising an annular seal means (30) characterised in that said annular seal means (30) has an inner surface (40) dimensioned to fit around said tube member (18) and an outer surface (46) dimensioned to fit within at least art upper end of said sleeve member (24), said inner surface (40) being provided with an inner fluid seal means (42,44) for forming a sliding fluid seal to the exterior (19) of said tube member (18), said outer surface (46) being provided with an outer fluid seal means (48,50) for forming a sliding fluid seal to the interior of said sleeve member (24) and the downhole tool sub-assembly (20) being located within an annular Volume (C) between the seal member (26) and said annular seal means (30), the Volume (C) being constant nothwithstanding movement of the tube member (18) relative to the sleeve member (24).
 2. A downhole tool assembly as claimed in claim 1 wherein said annular seal means includes a fluid passage linking axially opposite ends of said annular seal means, and fluid flow control means selectively operable either to open said fluid passage to the flow of fluid therethrough between said axially opposite ends of said annular seal means or to close said fluid passage to the flow of fluid therethrough between said axially opposite ends of said annular seal means.
 3. A downhole tool assembly as claimed in claim 2, wherein said fluid flow control means comprises a valve means fitted in said fluid passage or on one end thereof.
 4. A downhole tool assembly as claimed in claim 2, wherein said fluid flow control means comprises a plug insertable into said fluid passage for retention therein to block said fluid passage to fluid flow therethrough between said axially opposite ends of said annular seal means.
 5. A downhole tool assembly as claimed in claim 1, wherein said annular seal means comprises a body which is axially divided into a plurality of mutually securable segments whereby said annular seal means is assembled around a selected portion of said tube member of said tool assembly.
 6. A downhole tool assembly as claimed in claim 5, wherein said body is formed as a pair of hemi-annular segments mutually securable by latches or claims.
 7. A downhole tool assembly as claimed in claim 5, wherein gaskets are incorporated between mating faces of said body segments to inhibit fluid leakage.
 8. A downhole tool assembly as claimed in claim 1, wherein the axial end of said annular seal means which is uppermost and remote from said tool assembly in use of said protective arrangement is formed as a reamer or debris cutter to facilitate withdrawal of said protective arrangement from a downhole location subsequent to use of said tool assembly.
 9. A downhole tool assembly as claimed in claim 1, wherein the axial end of said annular seal means which is lowermost and adjacent said tool assembly in use of said protective arrangement is formed with dog clutch means selectively engageable with matching dog clutch means on said tool assembly to enable said annular seal means to be selectively rotated by rotation of said tube member of said tool assembly during withdrawal of said protective arrangement from a downhole location subsequent to use of said tool assembly.
 10. A downhole tool assembly as claimed in claim 1, wherein said inner fluid seal means comprises at least one elastomeric ring mounted in and protruding from a circumferential slot or groove extending circumferentially around said inner surface.
 11. A downhole tool assembly as claimed in claim 10, wherein said inner fluid seal means comprises a plurality of elastomeric O-rings each mounted in and protruding from a respective circumferential slot or groove each extending circumferentially around said inner surface.
 12. A downhole tool assembly as claimed in claim 1, wherein said outer fluid seal means comprises at least one elastomeric ring mounted in and protruding from a circumferential slot or groove extending circumferentially around said outer surface.
 13. A downhole tool assembly as claimed in claim 12, wherein said outer fluid seal means comprises a plurality of elastomeric O-rings each disposed in and standing proud of a respective circumferential slot or groove each extending circumferentially around said outer surface.
 14. A downhole tool assembly as claimed in claim 1, also comprising emergency release assembly in said tube member or in said tubular extension member thereof (if present) or therebetween (for example, in a further component or sub-assembly of said tool assembly).
 15. A downhole tool assembly as claimed in claim 14, wherein said emergency release assembly comprises a coupling means mechanically and hydraulically coupling parts of said tool assembly above and below said coupling means, said coupling means incorporating shear means rupturable to part said coupling means by an upward force applied through said tube member and in excess of a predetermined normal maximum upward force prevailing during non-emergency operation.
 16. A downhole tool assembly as claimed in claim 15, wherein said coupling means incorporates a vent opened by parting of said coupling means to vent said annular volume and so hydraulically release said annular seal means from its normal hydraulic retention in said upper end of said sleeve member.
 17. A downhole tool assembly as claimed in claim 14, wherein said tube member incorporates a longitudinal seal bypass means at a location on the exterior surface thereof normally below said annular seal means but capable of being lifted, by upward movement of said tube member with respect to said annular seal means, to a higher location in which said longitudinal seal bypass means longitudinally bypasses said inner seal means of said annular seal means to vent said annular volume and so hydraulically release said annular seal means from its normal hydraulic retention in said upper end of said sleeve member.
 18. A method of assembling and setting up the protective arrangement as claimed in claim 1, in conjunction with a downhole tool assembly of the kind specified, said method comprising the steps of providing said annular seal means and said members of said tool assembly, threading said tube member or said tubular extension member thereof (if present) through said seal member mounted in said sleeve member, locating said annular seal means around said tube member and inserting said annular seal means into said upper end of said bore of said sleeve member while said fluid passage is initially open so as to form an annular volume bounded externally by said bore of said sleeve member, bounded internally by the exterior of said tube member and by the exterior of said tubular extension member thereof (if present), bounded at one end by said seal member, and bounded at the other end by said annular seal means, filling said annular volume with a hydraulic liquid such as to exclude substantially all gasses from said annular volume, and operating said fluid flow control means to close said fluid passage to the passage of fluid therethrough between said axially opposite ends of said annular seal means whereby said annular seal means is retained in said upper end of said sleeve member substantially only by hydraulic forces resulting from the substantial lnvariability of said annular volume upon relative axial movement of said tube member and of said tubular extension member thereof (if present) with respect to said sleeve member and said seal member mounted therein.
 19. A downhole tool assembly comprising a sleeve member (24), a tube member (18) partially located within said sleeve member (24), a running tool sub-assembly (20) below the tube member (18) and a tubular extension member (22) depending from the running tool sub-assembly (20), characterized in that the external diameter (19) of the tube member (18) is equal to the external diameter of the tubular extension member (22).
 20. A downhole tool assembly comprising a sub-assembly (20) being located within an annular volume (C) externally bounded by a bore (25) of a sleeve member and internally bounded by an exterior surface (19) of a tube member (18) in conjunction with an exterior surface of a tubular extension (22) and the sub-assembly (20) linking them, and further bounded at a lower end by an annular seal (26) and at an upper end by a junk bonnet (30), characterized in that longitudinal movement of the tube member (18) together with the tool sub-assembly (20) and the tubular extension member (22) with respect to the sleeve member (24) and the seal (26) causes no change in the volume (C). 